Cover

One of the unique aspects of Dawn of War compared to other RTS games at the time of its release was the use of special areas of terrain designated as cover. By effectively utilizing cover a squad can fight for far longer than they could otherwise and even win when badly outnumbered.

Cover is applied on a unit by unit basis, not to the entire squad. Part of a squad can be under cover and the remainder can be exposed. A unit is affected by cover by simply standing on the cover terrain.

You can identify cover by simply moving your mouse over the terrain. When hovering over light or heavy cover, collectively known as positive cover, your mouse cursor will have an added blue shield icon with a dashed circle around it. When hovering over negative cover, your mouse cursor will have a cross mark like on a no smoking sign. The tooltips in the bottom right of the screen can also be used to identify which type of cover the mouse cursor is over.

What is Affected

All infantry and vehicles are affected by cover, although the bonuses differ for each unit type. However, there are several exceptions to this rule:

Air units are unaffected by cover.

Tau and Imperial Guard vehicles are affected by cover in the same way as infantry (minus the morale bonus of course). This includes everything from Chimeras to the Baneblade, Drone Squads to the Greater Knarloc, both positive and negative cover.

No Necron units are affected in any way by cover, positive or negative.

Melee

Although not technically cover, melee bonuses function in a similar manner and so are included here for comparison. All infantry, including Necrons, capable of engaging in melee combat receive these bonuses when fighting in melee and the target is also in melee combat. This means infantry do not gain melee bonuses when attacking buildings or non-melee infantry and vehicles. If the attacker is infiltrated but undetected, it will still gain melee bonuses but the target will not until it detects the attacking units and engages them in melee as well. Units also don't count as being in melee combat while performing sync kills, although they are invulnerable. Vehicles do not receive melee bonuses. As with normal cover, there are exceptions to the rule:

Negative Cover

Melee Combat

Ranged damage received: 77.4%
Ranged morale damage received: 10%

Tactics for Using Cover

Stacking Damage Reduction

Besides the already significant damage reduction provided by positive cover, the effects stack with other sources of damage reduction to make the affected units nigh-impervious to ranged attacks. For example, the Smoke Launchers on Space MarineRhinos reduce range damage for all units under the smoke by 50%. When used on units already under heavy cover, this makes the affected units take only 0.5 * 0.5 = 25% ranged damage. For units under light cover, they take 0.75 * 0.5 = 37.5% ranged damage.

Cover and Melee

Combining cover with melee units is especially effective, resulting in just 58.05% ranged damage in light cover and 38.7% ranged damage in heavy cover. The morale effects also synergize very well, doubling the morale regeneration of the units while simultaneously causing them to take only 10% ranged morale damage.

For this reason, engaging melee troops in heavy cover using a ranged focused army is a recipe for disaster. The ranged troops will be unable to cause any significant physical or morale damage to the melee units. This is particularly true if the melee troops can get into melee with a unit of yours that can likewise engage in melee, allowing them to get their melee cover bonuses. Constructing buildings in or adjacent to positive cover should also be avoided for the same reason. Any melee units attacking those buildings will be provided with strong defenses against your army while they rip apart your structures.

Setting Traps

Cover can also be used to set traps for your enemy. Heavy cover offers a very appealing prize for attacking units to take advantage of. Knowing this, placing mines in heavy cover can present an unexpected surprise for any attackers, especially since mines count as ability attacks rather than physical damage, allowing them to hit for full damage despite the heavy cover. Meanwhile your opponents can only slowly move out of the heavy cover due to the movement speed penalty.

This tactic can be further enhanced with turrets when negative cover is involved. While the enemy troops are forced to move slowly through the negative cover, they are prime targets for turrets as well as mines. The turrets will also do 10% extra damage to the enemy while they are stuck in the negative cover for an extra edge against them.